It is standard practice to provide devices for controlling the signals at a traffic intersection so that the traffic flows through the intersection in a preselected pattern. Such devices or controller are often located at the intersection and may be coordinated with similar controllers at other intersections by a master traffic controller. Although many intersections located or local controllers and their coordinating master controllers now employ solid-state digital technology, one of the most commonly used traffic controller is a relatively simple pretimed unit. These controllers have a preselected signal pattern which is processed repeatedly to control signal energization at an intersection. Similar pretimed controllers are located at other intersections. To coordinate these controllers for smooth traffic flow, the cycles are often offset by an amount or time determined by the desired traffic flow. To accomplish this offset coordination, synchronization pulses created by the master controller are directed to the various pretimed controllers at the intersections. These pretimed controllers generally involve the signalization at an intersection which includes a high volume street, known generally as the main street, and a lower volume street, known generally as the cross street. The ratio of time or cycle portion dedicated to the main street and the cross street is referred to as the split and is controlled at a given intersection by existing pretimed controllers. In summary, a local traffic controller of the pretimed type can include control of the offset, the split, the dial (timing plan) and cycle length.
For a number of years, the pretimed traffic controllers located at an intersection were electro-mechanical devices. Electric motor continuously rotated one or more interval controlling devices, referred to in the trade as dials. A single rotation of the dial was a cycle which had a length or duration determined by the speed of a drive motor and the ratio of gears connecting the drive motor and the rotating dial. To control the cycle length, the speed of the motor could be changed. More commonly, the gearing between the motor and the dial was changed to control the cycle length. The general concept is shown in Hendricks U.S. Pat. No. 3,047,338 as it related to a cycling of a master controller. The signal plan of the local pretimed controller was determined by a camshaft which operated the traffic signal lights in accordance with the position of the camshaft. The camshaft was indexed or stepped from position-to-position by tabs located at spaced positions on the dial.
Each index of the camshaft was referred to as an interval. Each interval would control the condition of the lights at the intersection in accordance with the position of cams on the camshaft, as they opened and closed switches used to energize the traffic lights. The length or time duration of each interval in the signal plan, as determined by the camshaft, was controlled by the time between a step starting the internal and the next step stopping the internal. The spacing of these steps was referred to as a percentage of a single rotation on the dial in a cycle. Thus, during the single cycle of a pretimed controller, the camshaft is indexed through a signal plan which controlled the various signal lights in accordance with a preselected, fixed pattern. The time of all intervals, when added together, equal the cycle length determined by single rotation of the dial. In more sophisticated electro-mechanical units, two or more dials could be provided, having different cycle length and/or different spacing of the tabs to index the camshaft through its signal plan. One of the dials could be used to change the split between signalization of a cross street and main street. In some controllers, more than one camshaft was provided. Each of these camshafts would have its own signal plan.
In summary, the standard, commonly used pretimed controllers included a camshaft which would determine the signal plan. A dial was rotated by a constantly rotation motor to control the cycle and the time during which the camshaft was in each indexed position or interval. This type of electro-mechanical device was generally fixed in operation and could not be varied substantially, except to control coordination, offset and similar features.
In recent years, traffic controllers have steadily progressed from electro-mechanical devices, to electronic devices and then to digital, solid-state devices. With the advent of microprocessors, the digital traffic controllers have been gradually shifting to programmable controllers with the executive program permanently stored in a PROM. Thus, traffic controllers now being produced are usually digital and programmed for process or control. Most of these devices are traffic actuated or modified. With the capabilities of complex programming, the units are not pretimed. This presents compatibility problems when the previous pretimed units are to be replaced by a controller of the present technology. Most of the local controllers with the programmable type are too expensive to be used as a pretimed unit or controller. In addition, such units have not heretofore been available as a direct replacement for a pretimed, electro-mechanical controller. For these reasons, relatively expensive programmable units are being employed at localities which require the only relatively simplified pretimed capabilities. In some instances, pretimed controllers of the electro-mechanical type are being used. These also present substantial cost. In either instance, the results are not satisfactory. An expensive programmed unit with too many variables in operation are not compatible with existing pretimed controllers. New electro-mechanical controllers are too expensive and involve technology not always wanted by a traffic engineer.
There is a need for a programmable pretimed controller. Efforts to develop such controllers have generally involved only units to provide in digitized versions of an electro-mechanical controller. This precludes any versatility and causes certain compatibility problems. Also, these units did not have any flexibility which could be obtained without complicated programming.